This proposal focuses on two aspects of the relationship between substance use and sexual behavior among teenagers and young adults. The first involves evaluating the extent to which the relationship between the use of such substances as marijuana, cocaine, and alcohol and various aspects of sexual behavior are causal. That is, does the use, of marijuana, cocaine and alcohol cause young people to initiate sexual intercourse at an earlier age, to be more likely to engage in sexual intercourse in the past month or past year, to be less likely to use condoms or other methods of birth control, to have had more sexual partners. or to have experienced an unplanned pregnancy? The second involves an investigation of the effects of state and local policies that attempt to reduce substance use on the outcomes just mentioned and on the incidence of sexually transmitted diseases (STDs), including acquired Immunodeficiency syndrome (AIDS). Examples of these policy variables are the prices of cocaine and marijuana (which depend on resources allocated to criminal justice), penalties imposed on users of these substances for possession, public spending for drug prevention, the price of alcohol (which depends on the rate at which it is taxed), and statutes pertaining to alcohol sales, advertising, and server liability. Five one individual-level panels, one individual-level survey of different teenagers pooled over five years, and two time series of state or city cross sections will be employed: the original National Longitudinal Survey of Youth (NLSY), the young adult sample of the NLSY, the new NLSY97, National Education Longitudinal Study of 1988 (NELS88), the National Longitudinal Study of Adolescent Health (Add Health), the Youth Risk Behavior Survey (YRBS), and two data series from the Centers for Disease Control and Prevention's (CDC) Sexually Transmitted Disease Surveillance Reporting System. Some specific questions to be addressed include: What are the effects of marijuana, cocaine, and alcohol use on a variety of indicators of sexual behavior and risky sex when these effects are obtained by multivariate statistical procedures such as instrumental variables and fixed effects that account for reverse causality and unmeasured variables'? How do these effects differ by gender, race, and age (teenagers versus young adults)? What are the effects of illegal drug and alcohol regulatory variables on sexual behavior, risky sex, and incidence rates of AIDS, chlamydia, and gonorrhea?